[d2n-analysis-talk] Scintillator Time Offset Checks/Beta Correction

[David Flay]

Hi all,

I have been working on checking the time offset, timewalk, beta, and beta
vs. track-x in the past week or so.

Today, I've made a correction to beta -- it was initially at ~ 0.93.

By putting in an arbitrary channel shift into the L and R TDCs of <one>
plane -- I used S2m -- we can move the beta distribution to center on 1. 
The way this works is that it shifts the time avg. of one plane -- but
does <not> affect the time offset between L and R -- because we are adding
the same shift to L and R -- hence, the difference between these two still
gives zero.  Since beta is constructed as the time-of-flight between S1
and S2m, this shift is reflected in the central value of beta.  I have
done this, and we get the result seen in the plot attached.

If we compare to the previous plot I sent for the same run, we see that
beta is now ~ 0.9924.  I think this is close enough.  However, while the
calculations of the time offset seem pretty good, I don't know if I can
say the same for the avg. time for S2m.  I'd like to take a closer look at
this, and maybe tighten it up some. (See second plot).

Also -- the resolution of beta is highly correlated with the calibration
of S1 -- so maybe it's worthwhile to run through the calibration on S1 as
well -- it should tighten up the structure you see on the higher end of
the beta distribution -- which to me, looks a bit funny.  Subsequently, I
also want to point out that the beta vs. track-x plot seems to get better.
 I'm thinking this will also improve with iterating the procedure on S1.

Other good news is that once I place a cut on beta > 0, the mystical
momentum dependence that we saw in the avg. time for S1 goes away --
confirming that those were due to cosmics.  Applying the beta cut to my
timewalk studies shows that the current timewalk polynomial factors in the
DB are good, as there is no timewalk effect. (I sent this info in previous
e-mails -- just wanted to consolidate the info here.)


Dave

-------------------------------------------------
David Flay
Physics Department
Temple University
Philadelphia, PA 19122

office: Barton Hall, BA319
phone: (215) 204-1331

e-mail: flay at jlab.org
            flay at temple.edu

website: http://www.jlab.org/~flay
              http://quarks.temple.edu
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